The present invention relates generally to the field of blowout preventers (BOPs) used with coiled tubing, and, more particularly, to a safety check valve adapted for use with a blowout preventer used with coiled tubing.
Coiled tubing has grown in popularity and acceptance in the oil and gas industry in recent years. Coiled tubing is typically inserted into a well bore by way of an injector, a tubing stripper, and a BOP stack. More often than not, the well bore is under live pressure when the coiled tubing is run into or retrieved from the well. When the tubing is moving, the pressure is retained between the well and the coiled tubing outside diameter by the stripper. When the tubing is not moving, or is out of the well bore, a BOP valve retains the well pressure.
Coiled tubing systems typically include a depth counter display provided to a coiled tubing unit operator in a control cabin. The depth counter indicates to the operator how far down the coiled tubing is in the well, and thus this display also indicates when the coiled tubing is about to exit the stripper. The normal procedure is to stop coiled tubing travel when the end of the coiled tubing is above the BOP, but has not yet exited the stripper. When the coiled tubing is stopped, the BOP valve is closed, the pressure between the BOP and the stripper is bled down, and the coiled tubing is then withdrawn from the well.
If the coiled tubing unit operator did not have an indication of the length of the coiled tubing, the tubing could be pulled out of the stripper with full well pressure behind it, causing an uncontrolled release of well fluids until the BOP could be shut. Such an event could even result in a catastrophic fire or a deadly hydrogen sulfide gas leak. Thus, it is critical for the operator to know the precise location of the end of the coiled tubing.
The depth counter commonly comprises a wheel in rolling contact with the coiled tubing and coupled to an odometer. If the wheel slips against a slick coiled tubing, then a false reading will occur. Furthermore, if the coiled tubing breaks down hole, the operator has no means of knowing where the break occurred and thus the length of the tubing remaining intact below the stripper is unknown. In either case, the operator may pull the coiled tubing out of the stripper, resulting in the escape of well bore fluids at full well pressure, until the BOP valve can be closed.
Thus, there remains a need for a fail safe device which will retain well bore fluids under pressure if coiled tubing is extracted from a stripper before the BOP valve is shut. The present invention is directed to solution to this problem in the art.
The present invention provides a biased or spring-loaded check valve between the stripper and the BOP stack. When no coiled tubing is in use, the check valve is shut against well bore pressure. When coiled tubing is inserted through the stripper and before the coiled tubing enters the BOP stack, the coiled tubing butts against the check valve and opens it, permitting insertion of the coiled tubing.